This proposal is to initiate a systematic investigation of the factors regulating sexual maturation in the female hamster. There are five major goals of the proposed experiments. First, to examine the role of the biological clock, as expressed by the daily rhythm of gonadotropin (GTH) release, in the maturation of reproductive function in the mid- to late-prepubertal period. This daily rhythm of LH and FSH release begins on day 16 or 17 of age and persists until the initiation of ovulatory estrous cycles, 2 to 3 weeks later. This characteristic of prepubertal development distinguishes the hamster from other, more extensively studied models, such as the rat. The proposed experiments will involve an examination of the changes in ovarian secretory function after initiation of clock-timed GTH release and manipulations that will delay or advance its onset. These experiments are tests of the hypothesis that circadian gonadotropin release is required for maturation of the reproductive system in the late prepubertal period. A second goal is to assess the role of the ovary in the initiation of clock function. Ovariectomy, steroid replacement, and ovarian transplantation will be used to test the hypothesis that ovarian secretions modulate the development or the expression of clock-timed gonadotropin secretion. Third, the importance of the prepubertal pineal melatonin rhythm to clock-timed gonadotropin release will be determined. Pineal function is crucial to reproduction in the adult hamster and the fact that the daily rhythm of pineal melatonin is initiated at the same time as that of cyclic gonadotropin release suggests that it may also regulate sexual maturation in the prepubertal animal. Fourth, the ontogeny of negative and positive feedback systems will be examined as a function of age in the prepubertal hamster. These experiments will result in information crucial to an assessment of the rate of development of the CNS in the female hamster. Fifth, the participation of prolactin in the gonadotropin-induced ovarian secretion in the late prepubertal period will be examined. The results of these experiments will provide a more cohesive picture of the process of sexual maturation in the female hamster, and, in particular, the importance of biological clock function therein. The hamster is demonstrably a superior model for the investigation of clock function. Furthermore, these investigations will be the first systematic study of sexual maturation in a photoperiodic laboratory animal.